Xylene (C6H4(CH3)2)
occurs in three isomers (o-, m- and p-) which vary
in the site of attachment on the benzene ring of
the two methyl groups. They are liquids at room
temperature and pressure, and are soluble in water
(134 - 230 mg l-1). They have moderate
log Kow values (2.77 to 3.20) and tend to volatilise.

Production of xylenes is primarily associated with
the petrochemical and coal industries, with most
being produced by the catalytic reforming of naphtha,
a derivative of crude oil fractionation. Most (c.
89%) xylene is produced as a mixture of isomers,
along with benzene and toluene, with subsequent
isolation of xylenes as required. Technical (mixed)
xylene contains typical proportions of o-, m- and
p- isomers of 20-24%, 42-48% and 16-20% respectively,
with 10-11% ethylbenzene as an >impurity=.

Xylene is produced widely in the EC at high tonnages
(total production in the range 500 to 1,000 Ktonnes,
according to the EU's
IUCLID database). In the UK, production of mixed
xylenes is significant, but of the isomers, only
p-xylene is produced in isolation in substantial
quantities. Estimated annual production capacities
in the UK for the early 1990s were 150 Ktonnes
mixed xylenes and 200 Ktonnes p-xylene, with major
production at only two sites (ChemInform 1992).

Most xylenes (>90%) are used in mixed xylene
as a solvent and a constituent (BTX - benzene-toluene-xylene)
of vehicle, aviation and other fuels. This latter
use has increased significantly in the EC with the
widespread introduction of unleaded petrols (Crookes
et al 1993). Globally, the increasing use
of vehicular transport in developing nations is
also increasing the use of BTX and, thus, xylenes.
The industrial importance of individual isomers
decreases in the order p-, o-, m-, with major uses
including the following (Micromedex 1996):

p-

Production
of polyester resins and fibres

Vitamin and pharmaceutical synthesis

Solvent, e.g. in insecticides

o-

Production
of phthalic anhydride

Solvent, e.g. in insecticides, pharmaceuticals

Intermediate in synthesis of dyes

Additive in motor fuels

m-

Production
of isophthalic acid

Solvent, e.g. in insecticides

Intermediate in synthesis of organics, dyes

Additive in aviation fuel

During their production and use, xylenes are released
primarily to the atmosphere. The principal anthropogenic
releases of xylene remain consistent, i.e.:

accidental and deliberate release of crude oils
and petrochemical products (including during refining
of crude oils and distribution and use of products);

production of xylenes and derivative chemicals;
and industrial and domestic discharge of solvents
and other products.).

Entry into water may be direct or via atmospheric
deposition, runoff and leaching.

Recorded levels in the marine
environment

Xylenes have been regularly reported in the oceans,
estuaries, precipitation, rivers, groundwaters,
potable sources and drinking water, as well as aquatic
sediments and biota (Hedgecott 1990, Crookes et
al. 1993). High usage and release, high mobility
in the atmosphere, and natural sources all contribute
to widespread occurrence in waters. Post-1988 data
for UK waters were summarised in Hedgecott and Lewis
(1997). Most of the data are for estuarine and coastal
waters, with xylenes apparently detected in relation
to industrial releases of xylenes and releases during
oil extraction and transport. None of the published
values exceed the concentration of 30 mg
l-1 of total xylenes which has been proposed
as the annual average EQS value.

Fate and behaviour in the marine
environment

Hedgecott and Lewis (1997) reviewed the fate and
behaviour of xylenes.

Atmospheric xylenes are subject primarily to photo-enhanced
oxidation by reaction with hydroxyl radicals, and
this (and other reaction processes) is enhanced
by nitric oxides and solids. Half-lives in the atmosphere
have been variously estimated between 0.83 and 29 hours.
Oxidation in water is considerably slower with estimated
half-lives between 30 and 300 days. Hydrolysis is
unlikely (Hedgecott and Lewis 1997).

Xylene molecules are relatively simple and biodegradation
is widespread in environmental media, although o-xylene
appears to be slightly more recalcitrant than the
other isomers. Groundwater inocula have been reported
to completely degrade low concentrations of xylenes
in 2 to 20 days (varying with pre-exposure) under
aerobic conditions (when dissolved oxygen levels
are not limiting). Although anaerobic degradation
has also been observed, this is somewhat slower.
Typically there is a substantial lag period of around
30 (m- and p-) or >140 (o-) days before significant
degradation in unacclimated aquifer material, but
with pre-exposure and adaptation, degradation can
be significant (>80%) after an additional 26
(m- and p-) to 100 (o-) days, although it may take
longer (Hedgecott and Lewis 1997).

A moderate tendency to sorb to organic solids is
suggested by the log Kow values of 2.77 to 3.20
and log Koc values of 2.1 to 2.5. Xylenes may sorb
to aquatic sediments but higher proportions remain
in solution; low sediment-water partition coefficients
of 8.9 for o-xylene and 10.5 for p-xylene have been
measured for the Tamar Estuary (Hedgecott and Lewis
1997).

Xylenes readily volatilise from water and this
is probably the major single removal process in
most surface waters, with a half-life of a few to
tens of hours, depending on the degree of mixing
(Hedgecott and Lewis 1997).

Effects on the marine environment

Toxicity to marine organisms

An exhaustive literature review on the toxicity
of xylenes to marine organisms has not been carried
out for the purposes of this profile. The information
provided in this section is taken from existing
review documents (Hedgecott and Lewis 1997). The
most sensitive groups of organisms have been identified.

Hedgecott and Lewis (1997) reviewed data on the
aquatic toxicity of xylenes. A previous review (Hedgecott
1990) found saltwater data were limited to acute
studies only, with the most sensitive of those species
tested being the bay shrimp Crago franciscorum
with 96 hour LC50s of 1.1, 3.2 and 1.7 mg l-1
for o-, m- and p-xylene, respectively (mean 2.0
mg l-1). The most sensitive fish
was the striped bass Morone saxatilis, with
96 hour LC50 values of 9.6, 7.9 and 1.7 mg l-1
for o-, m- and p-xylene respectively. These invertebrate
and fish results are very similar to the lowest
ones for comparable freshwater species.

Hedgecott and Lewis (1997) found few data had become
available since the previous review (Hedgecott 1990),
and again relate only to acute exposure. The lowest
effect concentration is an EC50 for the Microtox
bioassay (using Vibrio fischeri) of 8.5 mg
l-1 mixed xylene (Calleja et al.
1994). This is similar to a 15 minute Microtox EC50
of 9.2 mg l-1 determined previously for
p-xylene (Hedgecott 1990), and does not indicate
greater sensitivity than previously determined for
this or other saltwater species.

No data could be located for sediment-dwelling
organisms.

Bioaccumulation

Data summarised in (Hedgecott 1990) from fresh
and saltwater studies indicated that bioaccumulation
of xylenes was not significant, with BCF values
ranging from 1 to 15 in freshwater fish and 1 to
24 in saltwater fish and invertebrates, and uptake
and depuration both occurring rapidly. Hedgecott
and Lewis (1997) found few additional bioaccumulation
data had become available since the 1990 review;
reported BCFs from a few freshwater test with algae
are in excess of 200 but are based on dry algal
weight (Herman et al. 1991b). Thus, the new
data do not indicate higher bioaccumulation potential
than that indicated previously.

Xylenes in marine fish in Japan have previously
been implicated in tainting problems (Hedgecott
1990). Jardine and Hrudey (1988) determined a taste
tainting threshold of 9 mg kg-1
for p-xylene spiked into a freshwater fish (the
walleye, Stizostedium vitreum). Assuming
a fish BCF of 15, this implies that a water concentration
of 0.6 mg l-1 or above might
lead to detectable tainting of edible fish.

Potential effects on interest
features of European marine sites

Potential effects include:

toxicity of xylenes to invertebrates and fish
at concentrations above the proposed EQS of 30
mg l-1
(annual average) of total xylenes in the water
column.